1. Field of the Invention
The present invention relates to the manufacture of a forming die used to form optical glass devices such as objective lenses incorporated in an optical pick up. Particularly, the present invention is directed to the manufacture of a forming die in which the forming surface of a forming member can be coated with a film of aluminum nitride (AlN) or aluminum oxynitride (Al.sub.x O.sub.y N.sub.z) which provides a high surface roughness at relatively low temperatures so as to achieve a forming surface which has high adhesion, heat-resistance and low surface roughness and a small wettability to a forming material such as a glass.
2. Description of the Related Art
In the forming die used to form optical glass devices such as objectives, a certain type of film is generally coated on the forming surface thereof to improve the surface characteristics thereof, such as the die releasing characteristics. For example, Japanese Patent Laid Open No. 197430/1986 discloses a forming die whose forming surface is coated with a film made of aluminum nitride (AlN) so that it exhibits improved die releasing characteristics (wettability) and heat resistance at high temperatures. In this technique, the film of AlN is formed using PVD (physical vapor deposition) such as sputtering or ion plating.
However, in the deposition of a film made of AlN by the physical vapor deposition process, there is a limitation to the improvement in the surface roughness of the coated film.
Regarding deposition of a thin AlN film by the CVD (chemical vapor deposition) process, monthly magazine, "Industrial Material" (published by Nikkan Kogyo Shinbunsha) describes on page 55 of No. 9, Vol 31 that AlN can be deposited on a substrate of SiC at a temperature from 1200 to 1250.degree. C. by the CVD process using a mixture gas of aluminum chloride (AlCl3) and ammonia (NH.sub.3). In this described technique, however, deposition is possible only at high temperatures ranging from 1200 to 1250.degree. C. Furthermore, the films formed by the above technique have a rough surface. Particularly, deposition of AlN on the surface of a forming member made of a super hard material at such high temperatures, which is conducted when a heating type forming die is manufactured, deteriorates the super hard material. Furthermore, a forming die whose surface is rough cannot be used as the heating type forming die.
Substantially no research has been ever made on the deposition of aluminum oxynitride (Al.sub.x O.sub.y N.sub.z) which is known as excellent a material as aluminum nitride (AlN) in terms of heat-resistance and optical characteristics. In the reports exceptionally made on the research, however, the use of thermal CVD has been reported. This technique requires a substrate having a very high temperatures. Deposition of Al.sub.x O.sub.y N.sub.z on the surface of a forming member made of a super hard material at high temperatures, which is necessary to manufacture a heating type forming die, deteriorates the super hard material.